In recent years, a piezoresistance-type triaxial acceleration sensor for semiconductor devices using a MEMS technology (Micro Electro Mechanical Systems) has been developed as a light-weight and small-sized triaxial acceleration sensor which can be assembled into a portable instrument (refer to Patent Document 1, for example).
FIG. 36 is a perspective view showing a brief constitution of a conventional piezoresistance-type triaxial acceleration sensor, in which the reference numeral 201 denotes a silicon substrate, 201a denotes a supporting portion, 201b denotes a weight portion, and 201c denotes a displacement portion. The silicon substrate 201 is constituted by etching with the displacement portion 201c, the supporting portion 201a for supporting the displacement portion 201c and the weight portion 201b for deforming the displacement portion 201c. 
Further, piezoresistances R1 to R12 are formed on the displacement portion 201c. Upon application of acceleration to the silicon substrate 201, the weight portion 201b acts to deform the displacement portion 201c, depending on a direction and a magnitude of the acceleration. Then, stress is applied to the piezoresistances R1 to R12 to change resistance values.
FIG. 37A to FIG. 37C are circuit diagrams showing the wire connecting constitution of piezoresistances used in a conventional piezoresistance-type triaxial acceleration sensor. A wheatstone-bridge circuit constituted with piezoresistances R1 to R12 is individually provided by axial directions which detect the acceleration. Output voltage values Vx, Vy and Vz are proportional respectively to x-, y-, z-axis direction acceleration components.
Actual values of Vx, Vy and Vz of the circuit diagrams in FIG. 37A to FIG. 37C are expressed by the following.Vx=βxAx+Vox  (1)Vy=βyAy+Voy  (2)Vz=βzAz+Voz  (3)
where, Ax, Ay, Az denote x, y, z axis direction acceleration components, Bx, By, Bz denote sensitivities to Ax, Ay, Az, and Vox, Voy, Voz denote offsets present in Vx, Vy, Vz.
In general, sensitivity or offset varies to some extent, and in particular, there is often a case where the variation of offsets is not neglible. Further, in a piezoresistance-type acceleration sensor, the sensitivity and the offset are characterized by marked temperature characteristics. Additionally, the temperature characteristics of the offset often vary to a great extent.
In order to solve the above-described disadvantages, the following means is provided in a conventional acceleration measuring device (refer to Patent Document 2, for example). That is, as a factory default, the sensitivity and the offset are measured in a plurality of different temperature atmospheres, for example, at 0° C., 25° C. and 60° C. and storage means such as EEPROM is mounted on an acceleration measuring device to store measuring data.
Further, when an acceleration measuring device is used, an output correction circuit is mounted on the acceleration measuring device to make correction by calculating the variation of sensitivities and offsets contained in acceleration sensor output voltage and temperature characteristics on the basis of current temperature data and previously stored measurement data.
However, this type of a conventional acceleration measuring device is disadvantageous in the following points.
1) Measurement in a plurality of different temperature atmospheres and measurement of sensitivities will result in a great increase in process number, measurement time and cost of facilities.
2) Calculation of the sensitivity of an output correction circuit and temperature characteristics of offset makes the circuit constitution more complicated to eventually result in an increased production cost.
3) In order to calculate the sensitivity and the temperature characteristics of offset at an improved accuracy, it is necessary to increase measurement temperature points and further complicate a temperature-characteristics calculating portion in an output correction circuit. This is actually difficult in realization.
Further, in a conventional acceleration measuring device, the following solution means is further employed (refer to Patent Document 3, for example). For example, as illustrated in FIG. 38A to FIG. 38F, each time an acceleration measuring device is used, the attitude of an acceleration measuring device 203 is set by six different manners so that the acceleration detecting axial direction of the triaxial acceleration sensor 202 is parallel with the direction of gravitational acceleration g, to measure the output voltage of a triaxial acceleration sensor 202 six times, thereby acquiring the following output voltage data.    Vx1: Vx measurement value in a attitude shown in FIG. 38A    Vx2: Vx measurement value in a attitude shown in FIG. 38B    Vy1: Vy measurement value in a attitude shown in FIG. 38C    Vy2: Vy measurement value in a attitude shown in FIG. 38D    Vz1: Vz measurement value in a attitude shown in FIG. 38E    Vz2: Vz measurement value in a attitude shown in FIG. 38F
Sensitivity and offset data necessary for the output correction of a triaxial acceleration sensor are calculated by the following formulae.
                              β          x                =                                            V                              x                ⁢                                                                  ⁢                1                                      -                          V                              x                ⁢                                                                  ⁢                2                                                          2            ⁢            g                                              (        4        )                                          β          y                =                                            V                              y                ⁢                                                                  ⁢                1                                      -                          V                              y                ⁢                                                                  ⁢                2                                                          2            ⁢            g                                              (        5        )                                          β          z                =                                            V                              z                ⁢                                                                  ⁢                1                                      -                          V                              z                ⁢                                                                  ⁢                2                                                          2            ⁢            g                                              (        6        )                                          V          ox                =                                            V                              x                ⁢                                                                  ⁢                1                                      +                          V              X2                                2                                    (        7        )                                          V          oy                =                                            V                              y                ⁢                                                                  ⁢                1                                      +                          V                              y                ⁢                                                                  ⁢                2                                              2                                    (        8        )                                          V          oz                =                                            V                              z                ⁢                                                                  ⁢                1                                      +                          V                              z                ⁢                                                                  ⁢                2                                              2                                    (        9        )            
However, this type of a conventional acceleration measuring device is disadvantageous in the following points.
1) It is necessary to adjust the attitude of an acceleration measuring device several times to a predetermined direction each time it is used, which gives a great inconvenience to a user.
2) Further, it is difficult for a user to attain an accurate adjustment of the direction, with the acceleration measuring device supported by hand. The sensitivity and the offset calculated by the above formulae may have a greater error.
The present invention has been developed in view of the above-described disadvantages, an object of which is to provide an acceleration measuring device capable of acquiring the offset or both of the sensitivity and the offset necessary for correcting the output of a biaxial or a triaxial acceleration sensor by repeatedly acquiring output data of the biaxial or the triaxial acceleration sensor without consciousness of pointing the attitude of the acceleration measuring device in a specific direction.    [Patent Document 1] Japanese Patent Application Laid-open No. 2003-101033    [Patent Document 2] Japanese Patent Application Laid-open No. 6-331647    [Patent Document 3] Japanese Patent Application Laid-open No. 2004-93552    [Non-Patent Document 1] W. H. Press, S. A. Teukolsky, W. T. Vetterling and B. P. Flannery, Numerical Recipies in C, Second Edition, Cambridge University Press, USA, 1992, pp. 394-455    [Non-Patent Document 2] W. H. Press, S. A. Teukolsky, W. T. Vetterling and B. P. Flannery, Numerical Recipies in C, Second Edition, Cambridge University Press, USA, 1992, pp. 32-104